As optical disk devices projecting a laser beam emitted from optical pickup devices onto signal recording layers of optical disks to perform a signal reading operation and a signal recording operation, developments have been made for optical pickup devices capable of performing the signal reading operation for optical disks of the Compact Disc (CD) standard and the Digital Versatile Disc (DVD) standard and for optical disks of the Blu-ray Disc (BD) standard. In such an optical pickup device, a single housing accommodates laser diodes emitting laser beams of plural (three, for example) wavelengths corresponding to the respective standards and optical systems guiding the emitted laser beams to optical disks (refer to Patent Document 1, for example).
With reference to FIG. 7, a description is given of an example of the conventional optical pickup devices. This example is a so-called three-wavelength compatible optical pickup device corresponding to laser beams of three different wavelengths and includes two optical systems which collect laser beams by two objective lenses and guide the same to optical disks. FIG. 7 is a plan view for explaining a main portion of the optical pickup device in the housing.
A laser diode 101 emits a first laser beam which is blue. The first laser beam is transmitted through a first diffraction grating 102 and is incident on a beam splitter 103. The laser beam reflected by the beam splitter 103 is reflected by a reflection mirror 104 and is then transmitted through a first collimator lens 105 to be reflected by a first reflecting mirror 106 in a direction perpendicular to a signal recording surface of an optical disk (in a direction Df). The reflected first laser beam is transmitted through a quarter wave plate (not shown in the drawing) and is converged by a first objective lens for focusing the first laser beam to be projected onto an optical disk of the BD standard.
The first laser beam reflected by the optical disk returns the same path as the forward path. Part of the first laser beam is transmitted through the beam splitter 103 and is given astigmatism at a first astigmatism plate 108 to be projected onto a first photodetector 109 as a circular or elliptical spot.
A two-wavelength laser diode 111 emits a second laser beam as red light and a third laser beam as infrared light. The second and third laser beams are transmitted through a second diffraction grating 112 and is incident on a beam splitter 113. The laser beams transmitted through the beam splitter 113 are passed through a second collimator lens 114 to be reflected in the direction perpendicular to the signal recording surface of an optical disk (in the direction Df). While illustration is omitted, the second and third laser beams are passed through a quarter wave plate and are then converged by a second objective lens for focusing the second and third laser beams. The second laser beam is projected onto an optical disk D of the DVD standard, and the third laser beam is projected onto an optical disk D of the CD standard.
The second and third laser beams reflected by the optical disk D return the same path as the forward path. Part of the second and third laser beams is transmitted through the beam splitter 113 and is then given astigmatism at a second astigmatism plate 118 to be projected onto a second optical detector 119 as a circular or elliptical spot.
The first and second photodetectors 109 and 119 include light receiving units 109S and 119S, respectively and each develop a focus error signal, a tracking error signal, and a reproduced RF signal.
Each of the light receiving units 109S and 119S is composed of a sensor separated into quarters by two division lines DL, for example, and one of the division lines DL is set in a signal track direction (the direction Dt) of the optical disk. The first laser beam is given astigmatism by the first astigmatism plate 108 while being rotated for projection such that the long side of the elliptical spot S is inclined at 45° with respect to the division line DL of the light receiving unit 109S.
Similarly, the second laser beam is given astigmatism by the second astigmatism plate 118 while being rotated for projection such that the long side of the elliptical spot S is inclined at 45° with respect to the division line DL of the light receiving unit 119S.
The focus error signals for use in a focus control operation are generated from the spots S detected by the quadrant sensors 109S and 119S by an astigmatic method.
Patent Document 1: Japanese Patent Laid-open Publication No. 2008-123605